Solar heaters can heat water for many domestic, commercial and industrial needs. A basic feature of a solar heater is that it defines an enclosed liquid flow path, typically within tubing having good thermal conductivity; and the tubing is adapted to be exposed to the radiant heat of the sun. The tubing may be enclosed in a transparent housing, to minimize convective heat losses. Heat retaining material may surround the tubing, and/or black or heat-absorbing coatings may be on the tubing, for more effectively absorbing the radiant heat. A light collector device may be located adjacent the tubing for more effectively focusing the sun rays onto the tubing.
One configuration of the housing enclosure and/or light collector device is in the form of an elongated curved reflector, and a substantially straight run (or possibly a number of separate spaced runs) of tubing may be located at or near the focal axis of the reflector. Such a housing or collector device frequently is extended in a North-South alignment, and is pitched between 20 and 50 degrees above the horizontal, depending on the latitude. This orientation angles the collector so that it may be substantially normal to the sun's rays, as the sun moves across the sky during its daytime movement.
When the housing or collector device has no operating or moving parts, the solar heater may be considered as passive. A tracking system can be used to move the collector device about at least one axis, to track the sun during its daytime movement across the sky, for more effectively maintaining the sun rays properly focused on the liquid conveying tubing.
With the elongated collector configuration, and its low pitched orientation, both the passive and tracking collector take up considerable lateral area. Frequently the solar collector may be mounted on the roof of a building, to keep it out of the way, and also to locate it above small nearby foliage or other buildings that otherwise may block the sun. For these reasons also, a preferred location of the solar collector would be at the South side of a building; but when this also is at the front of the building, the solar collector may be conspicuously visible and possibly unsightly.
Tracking solar heaters operate with high thermal efficiencies, generating large thermal outputs with relatively short lengths of liquid conveying tubing. However, the moving parts and the needed controls for moving the parts, add to the costs of installation and continued operation.
Another form of a passive solar heater may arrange the liquid conveying tubing as a three-dimensional array. However, known tube arrays have drawbacks that may not fully utilize the radiant energy, and provide low thermal outputs or low outlet temperatures. Contributing conditions include that the sun rays strike only a few outer passes or runs in the tube array, and the sun is blocked from the remaining tubing; and/or the sun rays, throughout the course of the day, may radiate against the tubing at small angles, to yield high reflective losses; and/or the liquid flow sequence through the tube array may not provide the hottest temperatures at the heater outlet.